The present invention relates to a method for adjusting signal levels of image signals and image read apparatus adopting the method and, more particularly, to a method, in which light reflected from or transmitted through an original image are separated into color components, for automatically adjusting conditions for reading the original image depending upon the type of the original image so that the original image is properly expressed by image signals within a predetermined signal level range and an image read apparatus, such as an image scanner, adopting the method.
When adjusting image data of an original image inputted by a scanner so as to be within a signal level range which can be expressed on hard copies, a method of generating tone histograms of the image data and determining the proper signal level range by analyzing the histograms is generally used (Summary of the Digital Photo Print System "Frontier", Material for The 4th Camera Technology Seminar in Meeting of Japan Photograph Society, pp. 13-16).
The most fundamental technique of the method is to convert the distribution of the obtained histogram into a balanced distribution as disclosed in the U.S. Pat. No. 4,745,465. Further, in the U.S. Pat. No. 4,639,769, means for expressing an image in good contrast by changing mapping of each pixel data based on a histogram is disclosed.
Further, in a case of separating a color image into primary color components to generate digital data, by performing the aforesaid conversion of distribution on each of the color components, color bias of the original image, e.g., color cast in a color photograph, taken under fluorescent light, other than daylight, is compensated, thereby a final image is expressed in a desired contrast.
However, in the aforesaid conventional method, the signal level range is determined without considering characteristics of an original image. Furthermore, whether an imbalance in tone and color of the separated color components is due to color cast of the color image to be corrected, caused by light source other than daylight, or due to the original colors, of an object of the image, which are not to be corrected is not distinguished for determining the signal level range.
Accordingly, in a case where vivid colors fill most of the original image, a problem in which vividness of the colors is easily lost arises. Further, when the colors of the original image are biased, a phenomenon, called subjective color failure, occurs.